Phosphorus present in domestic wastewater from the use of laundry and dishwasher powders as well as human waste is an important macro-nutrient for plant and micro-organism growth. The discharge of large quantities of phosphorus into natural receiving sources produces the growth of algae and results in eutrophication of lakes and streams. Algae may grow at levels as low as 0.05 mg/L orthophosphate (PO43−).
The phosphorus concentration in effluent discharging from a wastewater treatment plant (WWTP) is usually within the range of 3-7 mg/L, which substantially comprises orthophosphate (“Phosphorus removal in low alkalinity secondary effluent using alum” R. J. Banu; K. U. Do; I. T. Yeom Int. J. Environ. Sci. Tech. 5 (1), 93-98, 2008, which is incorporated herein in its entirety by reference). It is often necessary to reduce the concentration of phosphorus in secondary wastewater to prevent algal blooms. Accordingly, consent by regulatory authorities to discharge effluent into a water environment usually requires a limit on the concentration of phosphate.
Aluminium salts, which are used as a coagulant in water and wastewater treatment plants, also precipitate phosphate as aluminium phosphate from the treated water prior to discharge. Chemical removal of phosphorus by the addition of an aluminium salt to precipitate the corresponding phosphate salt is one of the most reliable and well-established processes practiced by a WWTP.
The resulting waste sludge separated from the treated water contains high levels of organic material as well as the insoluble aluminium phosphate and other inorganic salts, such as aluminium hydroxide.
Known methods for recovering aluminium and phosphate from sludge for reuse generally employ acid extraction techniques using sulfuric acid, for example. These processes are, however, non-selective and can lead to excess contamination of the recovered aluminium compounds. Other techniques that have been employed include ion exchange and ion selective membranes.
Accordingly, it is an object of the present invention to go some way to avoiding the above disadvantages; and/or to at least provide the public with a useful choice.
Other objects of the invention may become apparent from the following description which is given by way of example only.